Help!! Will taking Plan B at 3 weeks PP (EBF) cause my period to come earlier than it would have naturally?

I feel ashamed to write this. SO and I planned on using a condom but we got carried away and did not use one. I'm 3 weeks postpartum, and completely breastfeeding my baby. I haven't had a PP period yet. I know fertility increases immediately following birth. I don't know if I'm ovulating or not. They say breastfeeding can delay ovulation but I'm not too keen on relying on that. With my first baby, I got my first PP period when he was 2 months old. 3 years later, I got pregnant with my second baby the ONE time I didn't use any birth control or protection. I'm scared that I'm extremely fertile. I'm going to go to the store and buy some Plan B today.